As a result of the vulnerability of wild cycad populations to human impacts, including collection for commercial horticulture, the Cycad Specialist Group of the International Union for Conservation of Nature (IUCN) recommends concerted efforts to improve propagation and cultivation of cycads (Donaldson et al., 2003). With this conservation concern in mind, important progress has been made in cycad horticulture (Chavez and Litz, 2007; Dehgan, 1983, 1999; Dehgan and Johnson, 1983; Witte, 1977), but to date, no studies appear to have been published investigating the effects of different substrates on cycad growth and development. The objective of this study was to determine whether use of 100% inorganic substrates will lead to an improvement in germination or growth of rare Zamia spp. in containers.
Providing the proper balance of aeration, water retention, nutrient-holding capacity, and decomposition rate are among the key factors that must be considered in evaluating a cycad container substrate. Excellent drainage is especially crucial, because cycads often do best in “sandy gravelly” soils (Whitelock, 2002). As a result of the slow growth of cycads, the ability of a substrate to retain its physical properties over time is crucial, especially in a hot, humid environment such as south Florida that accelerates decomposition of organic materials. Drainage of organic container substrates can decline considerably over time as the substrate decomposes (Bilderback et al., 2005). Thus, it seems likely that an ideal long-term cycad substrate would be inorganic and resistant to decay or at least contain a sufficiently high proportion of inorganic materials to retain drainage even after the organic components have begun significant decomposition.
The Montgomery Botanical Center (MBC) is a 120-acre botanical garden in Miami, FL, that specializes in conservation horticulture of cycads and palms. Montgomery currently cultivates two-thirds of extant cycad species and expends considerable effort propagating and growing cycads in containers as well as in the ground. Historically, MBC has used a horticultural mix comprised of equal parts organic soil conditioner (Fafard Organic Soil Conditioner; Conrad Fafard, Inc., Agawam, MA), silica-based coarse building sand (6/20 grade; Florida Silica and Sand Company, Ft. Lauderdale, FL), and expanded clay pellets (Hydroton® 8/16 mm grade; Ökotau Easy Green GmbH, Eschborn, Germany) as a substrate for both seed germination and nursery container culture of cycads.
In the current study, two other substrates, Turface®, a calcined montomorillinite clay (Turface® MVP®; Profile Products LLC, Buffalo Grove, IL), also known as arcillite, and coarse silica sand (6/20 grade; Florida Silica and Sand Company), were selected for comparison with MBC's horticultural mix (referred to as “cycad mix”) to evaluate the effects of inorganic substrates versus a typical mixed organic–inorganic substrate on germination and early seedling growth. Sand and Turface® have both shown considerable promise in hydroponic applications, and some horticulturists have used them successfully for growing cycads and succulents. As a result of their physical and chemical stability, these products are of interest to evaluate for their potential as long-term substrates for cycad culture.
Turface® is a calcined montmorillinite clay soil conditioner designed for use as an amendment or top-dressing for turf in sports fields. It is meant to hold moisture and nutrients, increase drainage, and reduce compaction. Calcined clay has long been known to be an excellent substrate for growth of experimental plants in hydroponics (Jaeger, 1981), partly because it allows easy separation of the substrate from the root system (Hiller and Koller, 1979) while also supporting good growth. Warren and Bilderback (1992) and Owen et al. (2008) found that calcined clay reduces water use and increases fertilizer efficacy in container production when used as a substrate amendment. Turface® has excellent drainage, porosity, and water-holding capacity (Table 1), is mechanically stable, and has high cation exchange capacity as a result of its montmorillinite clay makeup (Warren and Bilderback, 1992).
Physical properties of substrates used for growing Zamia spp.: Montgomery Botanical Center cycad mix, sand, and Turface® MVP®.z
Coarse silica building sand is often used as a germination substrate. One grower reported exceptional growth of cycads in 100% sand (T. Broome, personal communication), including difficult Macrozamia species that are especially sensitive to lack of drainage (Broome, 2006). The excellent drainage and inertness of sand render it promising for cycad culture. Sand has long been used as a standalone substrate in a very broad range of hydroponic research (Hewitt, 1966).
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